1. Field of the Invention
This invention relates to projection lenses, more particularly to projection lenses suitable for projecting a magnified image appearing on the cathode ray tube (CRT) of a television set on a separate large screen.
2. Description of the Prior Art
Generally, it is very hard to provide CRTs with a screen of a large size from the points of view of manufacturing difficulty and safety. Consequently, a magnified image is never displayed on the screen of a television set. In order to display a magnified image, there are heretofore well known projection television systems, which comprise a projection device and a television set combined. Such projection television systems include three single color CRTs for blue, green and red, a large screen, and three projection lenses each magnifying and projecting a single color image appearing on each CRT on the screen so as to compose the magnified single color image constituting the complex color image thereon.
Since a single color image appears on each CRT described above, it is not necessary to use an achromatic lens as the projection lens. On the other hand, it is preferable to use a lens with an increased aperture, that is a high speed lens, because of the requirement of high magnification power. Nevertheless, heretofore well known projection lenses merely have relative focal-length-to-aperture ratios (F numbers) between 1.2 and 1.0 at best. Moreover, it is a requirement for the economical production of projection lenses that the number of lens elements should be decreased not only for reducing the cost but also for making the projection television system compact. For this reason, it is essential for the projection lenses to have a reduced F number (F.sub.No), an angle of view as large as possible, and a low cost.
An example of a projection lens satisfying the requirements mentioned above is disclosed in U.S. Pat. No. 4,300,817, in which the projection lens comprises from the screen side a modified Schmidt plate for correcting aberrations mainly due to aperture, a biconvex lens of positive power providing substantially all the positive power of the overall lens, and a negative lens serving as a field flattener for correcting Petzval curvatures of the modified Schmidt plate and the biconvex lens. However, the projection lens of that patent does not correct aberrations sufficiently when the angle of view is larger than 50.degree.. Moreover, there still remain practical problems, namely, the manufacturing cost of the lens, and the difficulty of making a projection television system compact.